1. Field of the Invention
The present invention relates to a cooling system of an internal combustion engine having a turbo-charger. More particularly, it relates to a system for cooling the turbo-charger by circulating a part of the engine cooling water through a cooling water circulation passageway of the turbo-charger.
2. Description of the Related Art
A turbo-charger for an internal combustion engine includes an exhaust turbine impeller which is exposed to and heated by the high temperature of engine exhaust gas passing through the turbo changer. Therefore, the shaft bearings of the turbo-charger are also heated because of the heat transmitted from the turbine impeller. A bearing housing of the turbo-charger is, therefore, usually provided with cooling water jackets in the vicinity of the turbine impeller, and when the engine is running a part of the engine cooling water is forcibly circulated therethrough to cool the bearing portions of the turbo-charger. Namely, a cooling water passage is branched from the downstream side of a water pump to introduce the cooling water into the water jackets. Therefore, when the engine is running, the cooling water is compulsorily fed into the cooling system of the turbo-charger. However, when the engine is stopped, the performance for cooling the turbo-charger depends largely on a level at which the turbo-charger is positioned, since the bearing portions thereof are then cooled by a so-called natural or gravity circulation. Consequently, if the turbo-charger is positioned lower than the cylinder head of the engine, a natural circulation is established through conventional piping for the cooling water, which cools the the interior of the engine as well as the turbo-charger. However, if the turbo-charger is positioned higher than the engine cylinder, the cooling water may be boiled in the water jacket of the turbo-charger, thus filling the jackets with vapor (gas) which will prevent the circulation of the cooling water, when the engine is stopped after having been driven in a heavy load condition. Therefore, positioning the turbo-charger at a higher level than that of the engine with conventional piping for circulating cooling water causes an increase in the temperature of the bearing portions and a deterioration in the lubricant.
To avoid the above-mentioned problems, an electric water pump can be added exclusively for the cooling water piping of the turbo-charger, and this water pump is driven after the engine is stopped. This solution may provide effective cooling, but the electric water pump is very expensive per se, and the capacities of the alternator and battery must be increased to cope with the extra load incurred thereby.